Since the announcement of electroluminescent light emission from polymers by Burroughes et al (1) in 1990 the potential application of light emitting polymers (LEPs) in flat panel display devices (FPDs) has been investigated by numerous groups for both full color prototype display devices (2,3) and initial, monochrome commercial products. FPDs using LEPs have several potential advantages over the now well established liquid crystal display (LCD) technology. Among these advantages are: reduced form factor, reduced power consumption, increased brightness, reduced manufacturing costs, improved daylight readability, increased contrast and improved color purity (4). The construction of an FPD comprising LEP technology is very simple when compared to that used in the construction of an LCD-FPD, which can be readily illustrated by comparing FIGS. 1 and 2, which show exemplary materials used in the construction of a typical device.
Despite the projected economic advantages offered by the possible adoption of LEP technology, the development of devices using LEP technology has been hindered by a number of technical challenges. At present, engineering challenges associated with the scale up of laboratory prototype processes for large scale manufacturing limit the economic efficiency. It is anticipated that this difficulty will be overcome in time. A more serious issue is the persistent problem of device operating longevity. Device lifetime, which is limited by the slow degradation of both the interfaces between layers and chemical changes in the emissive material, is currently too short. In general, these effects vary for the different materials used to generate the primary colors, leading to an unacceptable variation in the ageing of the sub-pixels in a full color display (red lifetime≠green lifetime≠blue lifetime). The development of a long-lived and efficient blue emitting system, in particular, has remained a difficult problem for many years. Finally, an acute susceptibility to degradation by environmental moisture and oxygen requires the use of packaging techniques that constrain the choice of materials that can be used to fabricate the devices. Despite these significant frailties, LEP technology remains the subject of a great deal of research and development effort around the world, as this approach to organic light emitting device (OLED) technology offers the distinct possibility of ‘wet processing’, that is, ink jet printing or spin coating (5). These process technologies are well established and capable of delivering highly accurate and reproducible results at very low unit costs.
The most commercially successful of the various OLED technologies is the multilayer approach using small molecule fluorescent (SMF) technology that was originally reported in the seminal publication by Tang and van Slyke of the Kodak Corporation in 1987 (6), which is exemplified by the work of Kafafi et al (7). The organic compounds used in SMF devices exhibit the same environmental frailties seen in LEP materials, that is, they are very susceptible to degradation when exposed to either oxygen or moisture (8). The SMF approach to OLED utilizes a multi-layer or stacked approach of organic materials, with each layer fulfilling a particular role. The layers are: (1) an indium-tin oxide (ITO) anode; (2) a hole injection material; (3) a hole transport material; (4) an emitter; (5) an electron transport/hole blocking layer; and (6) a reflective metal cathode.
Other than the acute environment fragility observed in SMF based devices, this form of OLED technology is limited in its efficiency in converting electrical energy into light (electroluminescence efficiency), which is theoretically limited to 25% (9).
The potential quantum efficiency that can be attained using the small molecule approach has been enhanced by the development of triplet emitters or small molecule phosphorescent (SMP) technology that was developed initially by Forrest et al (10) and later commercialized by Universal Display Corporation (UDC). SMP emitters incorporate transition metal-atom-containing species that convert a high fraction of the input electric charge to emitted light and this form of technology possess a theoretical efficiency of 100%. Examples of SMP materials are adducts of the mercury trifunctional Lewis acid trimers and the arene compounds pyrene, naphthalene, and biphenyl (11). These adducts exhibit bright red, green, and blue phosphorescent emissions respectively in the solid state.
Despite the commercial success of SMF-OLED and the increasing success of SMP-OLED based devices, neither approach offers a satisfactory solution to the differing lifetimes observed in the various colors. SMF materials exhibit long lifetimes in the blue region of the spectrum and short lifetimes in the red region, while the opposite can be said for SMP materials, thus they cannot be viewed as being viable as a stand alone solution for high end FPDs, where lifetimes of at least 20,000 hours are a fundamental requirement. It is important to note, however, that in this common industry target “lifetime” is defined as the time until a 50% drop in the initial luminance of a device has occurred. Such a drop would produce an unacceptable differential ageing between neighboring pixels in displays in which persistent images or pull-down or pop-up text based screens are used. An additional limitation of both SMF and SMP-FPDs is that they are produced using organic vapor deposition (OVD) processes, which are expensive and arguably limited in the size of the substrate that can be used in the production process (12).
The multi-layer approach developed by both Kodak and UDC has undoubtedly paved the way for the expansion of OLED based technologies, which is expected to reach a market value of US $2.3 Billion by end of 2008, but if this vision is to be realized, further developments in OLED technologies must be made. The further expansion of OLEDs beyond niche applications such as intermittent use subdisplays in cellular telephones will not happen unless dramatic progress is made in the operating lifetime of the emissive pixels, especially blue.
LEP OLED technology offers potential economic advantages as well as a much higher efficiency when compared to SMF and much lower turn on voltages (13). When the electrons and holes injected from the opposing device electrodes meet in the bulk of the emissive layer, they combine to form a charge-neutral singlet or triplet excited state (exciton). The decay of that excited state results in the emission of light. During the charge-recombination process, the spin directions of the electrons involved can be oriented in one of four possible combinations, each with an equal statistical likelihood. The first pattern, the light-emitting ‘singlet’, can have only one of the four possible spin combinations. The other, a ‘triplet’, can have three different combinations. Thus, spin statistics predict that a singlet state will be formed in only 25% of all charge recombinations. Brédas (14) has shown theoretically that systems built from long polymer chains should be able to increase the percentage of light-emitting singlets to as high as 50%, which represents a 100% increase when compared to SMF. This increase in efficiency is believed to be a consequence of increasing molecular weight and as a result triplets take more time to convert to neutral excitons. This apparent increase in the decay time of the triplet state allows the unfavorable triplet state to ‘convert’ to a singlet, while singlet conversion to excitons remains rapid. As a result of this slowed decay, spin statistics are biased in favor of singlet formation and the resulting increase in efficiency. The primary limitation to the average molecular weight is solubility, that is, increasing molecular weight decreases solubility, which could potentially increase the difficulty of the subsequent processing of the polymeric materials. An alternative approach to improved efficiency in LEPs is being investigated by several groups, where they have reported improvement in the efficiency of an LEP by dispersing a phosphorescent ‘dopant’ material into an LEP ‘host’, with the result that it is possible to use all the excited states, both singlet and triplet, for light emission provided that the triplet energy gap of the host is higher than that of the guest (15).
As well as possessing a great deal of potential in ‘traditional’ display applications, LEPs possess the potential of being applied to flexible displays, which is an area of display development that is attracting a great deal of research effort (16).
It has been shown that the lifetime of an LEP-FPD is directly correlated to the applied voltage, that is, the higher the voltage the shorter the lifetime, thus in order to achieve the desired high brightness of modern applications, the lifetime of the device is by default, compromised (17). While significant strides have been made in the development of new polymeric materials, their lifetime is still not sufficiently acceptable for use in commercial products, particularly for the blue emitting materials. The blue emitting LEP materials tend to posses a wider band gap that tends to lead to a lower highest occupied molecular orbital (HOMO) level, which results in an energy-offset from the work function of the ITO electrode (18). Thus, successful development of blue-emitting LEP devices must include alternative approaches that address this offset in energy between the conducting material (for example ITO) and the blue-LEP material.
One such approach that has been investigated, particularly with respect to SMF technologies, is the use of self assembled monolayers (SAMs) in an effort to modify the field distribution in the vicinity of the anode to facilitate hole-injection (19-23). The use of SAMs has been reported to increase the internal efficiency of the emitting-layer; however, the previous approaches have not been adopted in the manufacture of OLED devices because the device performance was not improved sufficiently or the SAMs produced contained defects that result in inconsistent device performance, among other problems.
An improved LEP device is needed.